Pleuropneumonia is a major respiratory disease in pigs and causes severe economic losses in pig farming in many countries including the United States and Canada. The disease is caused by the bacterium Actinobacillus pleuropneumoniae (previously also referred to as Haemophilus pleuropneumoniae) and is considered to be one of the most important disorders of the bronchial tubes in pigs. Frequently, the disease is fatal. Actinobacillus pleuropneumoniae is known to exist in twelve infective serotypes.
Since pleuropneumonia can be induced by inoculating pigs with sterile culture supernatants of A. pleuropneumoniae, extracellular toxic proteins are assumed to be involved in the development of the pneumonic lesions. There is growing evidence that qualitative or quantitative differences in toxic activities exist between the twelve serotypes of A. pleuropneumoniae. Hemolytic and cytotoxic activities have been reviewed by T. A. Bertram, Can. J. Vet. Res. 54: 553-556 (1990). Two different hemolytic activities were reported by Frey and Nicolet, J. Clin. Microbiol. 28: 232-236 (1990), whereas four antigenically different activities were distinguished by Kamp and Van Leengoed, J. Clin. Microbiol. 27: 1187-1191 (1989). Whether such activities are functions of one or more molecules is not known.
Vaccines proposed thus far for preventing infections by Actinobacillus pleuropneumoniae are mostly based on whole live cells, attenuated cells, lysates, culture supernatants, or extracts of A. pleuropneumoniae. WO-A-80,02113 (or Canadian Patent 1,189,790) teaches a vaccine for controlling pleuropneumonia in pigs, containing A. pleuropneumoniae cells, cell fragments etc. and, as an adjuvant, material derived from Bordetella pertussis. EP-A-420,743 proposes a vaccine containing inactivated toxin of serotype 1 and optionally an inactivated toxin of another serotype of A. pleuropneumoniae; it provides protection against serotype 1 and partial protection against other serotypes. EP-A-354,628 discloses a universal vaccine against A. pleuropneumoniae, which contains extracellular proteins from two different serotypes, and is effective against all A. pleuropneumoniae serotypes. Although these known vaccines provide protection against some or even all of the field strains of A. pleuropneumoniae, the active compounds are not known. As a result, control, verification, and standardisation of vaccines is difficult, since the ratio between active components cannot be optimized and inactive and sometimes adverse components are always present in the vaccines.